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Sometimes you don't have good contact between the test screws. I usually use a pocket screwdriver and put it between the terminals on the back of the terminal strip. I use the step method, short between the test terminals one step at a time. If the unit is on a Trane Tracer system, the heat or cooling could be disabled on the front end. If it is, the test mode won't bring it on.

i built my own using products from digkey...i cost me about $75 which is pretty expensive for a regular ol' box but i made it really nice. it has a ball bearing selector switch, is water proof and can survive a drop off a roof.

If anyone is going to work on this type of equipment an trouble shoot a Microtel or Reliatel board than this is the best way to accurately know the mode which you are in.

Sure everyone knows how do perform a "Step" test, but it just not good enough for in depth trouble shooting.
Any one who has worked on these units when an erratic condition exist will tell you it can be time consuming.

Some get lucky for the most part, but when the components have failed or the performance has degraded such as an outside air sensor or mixed air sensor than their can be a lot of wasted time trying to understand the symptoms.

Its one of the correct methods of trouble shooting as recommended by the manufacture. Besides that there are many other and more important uses. If working on chillers or any other type of control work, this is a MUST have tool.

I understand those who don't have one have no other choice but to "Step" into problems, but there is a better way.

But I am still learning and looking for a new mentor.
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In a strict sense troubleshooting is not part of the repair..........understand the symptoms and you will find a solution.

I am unfamiliar with this measurement tool, can you explain what it does exactly as you troubleshoot??

thanks

When working with a Trane unit that has the capability of a manual test such as on a Microtel or Reliatel board it will put you in the exact mode of operation which you want to test.

As an example when testing a unit with a two stage heat and a VFD, the "STEP' method does not allow enough time for the VFD to ramp up so you must leave it in this step mode long enough for the control to react. It also take about one minute for the combustion blower to switch from high speed and then to low fan speed. And this is a must check step as these units have inherent concerns. However when a problem exist such as a open limit or pressure switch there will be times when the control will skip that mode. Many times you will have to cycle the test over and over again and carefully follow the steps.

It is also necessary to go through the unnecessary steps which are working correctly. A decade box will within a few seconds put you in the exact mode based on the variable resistance method. It will remain in this mode until either you reset the power or the elapsed time has passed. This will eliminate any guess work whatsoever. That's it...leaving you to concentrate on the problem.

Also you can immediately cycle stages on and off from heat to cool or any mode you wish. No waiting at all...just go to work.

Now if you got 20 units to PM good luck waiting for the step method.

But that not just it. You can test the economizer function or the ECA module when experiencing problems by giving it a false OSA reading by using the correct temperature/resistance value to demand the unit to economize.

There are many more reasons but you get the idea.

As for a chiller I have had good results trouble shooting temperature input on the clear language display to simulate temperature.
This is also invaluable when testing an EXV and demanding it to position, but that's not for the faint of heart and I am sure I will here some slack on this. But I am not the one changing them out when problems come up either.